Each year, millions of dry metric tons of biosolids, the nutrient-rich byproduct of municipal sewage treatment, are spread across US soils. These biosolids, often referred to as sludge, have been promoted as a natural alternative to petrochemical fertilizers, offering a sustainable method of nutrient recycling. However, many of these biosolids are tainted with PFAS, creating significant environmental issues.
Fortunately, scientists are actively seeking solutions. Environmental scientist Jake Thompson from Yale University highlights the issue’s impact, stating, “This issue – the contamination of agricultural land with PFAS – disproportionately hurts small farmers and organic farmers, who used fertilizers with PFAS before the full extent of the problem was known.”
Thompson and his team have introduced a novel remediation method detailed in the journal PNAS, which aims to lower PFAS levels at a fraction of the current costs. According to a 2001 National Sewage Sludge Survey by the US Environmental Protection Agency (EPA), 2,749 to 3,450 kilograms of PFAS, known as ‘forever chemicals’, are found annually in US biosolids, with about half applied to agricultural land.
PFAS are notorious for their persistence, as the carbon-fluorine bonds in these molecules are incredibly strong, allowing them to remain intact for thousands of years. When applied to soil, PFAS can accumulate, leaching into waterways or being absorbed by plants, potentially entering the food chain.
This is alarming given the growing evidence linking PFAS exposure to health risks. Laboratory and animal studies suggest PFAS may contribute to cancer, and issues with pregnancy and reproduction. Human studies have also identified associations between PFAS and certain cancers, cardiovascular risks, multiple sclerosis, and fertility problems. While many of these studies are in early stages, the potential danger of PFAS accumulation in agricultural soils is becoming increasingly clear.
Thompson notes, “Though this study is US-focused, based on the available data, PFAS are a challenge globally.” Addressing this contamination is daunting, as existing methods to remove PFAS from US farmland cost between $800,000 to $1.6 million per hectare, amounting to an estimated $8 trillion nationwide.
In contrast, the new method proposes a cost of about $29,000 per hectare, assuming the process is repeated for up to 20 years. (Yale News)
According to researchers, this approach also enhances soil health, unlike current methods that involve excavating and incinerating soil, which degrade topsoil and emit significant carbon dioxide.
The process begins by adding crushed alkaline rock to the contaminated fields. Thompson’s team used basalt in their models, though limestone could also be viable. “The application of alkaline rock amendments to soil (or enhanced weathering) elevates soil pH to a target pH of 7, leading to an increase in mobility and bioavailability of key PFAS such as perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA),” the authors explain in their study.
Plants like hemp and perennial grasses, known for their ability to absorb PFOS quickly, are cultivated in the contaminated soil, then harvested and subjected to pyrolysis (a type of heat treatment without oxygen) to create biochar.
“Harvested biomass converted to biochar and reapplied to the field can also serve to immobilize residual PFAS, thereby reducing their transfer into forage crops,” the team adds.
There are still many unknowns around the fate of PFAS during pyrolysis – a recent study shows that a temperature of at least 800 °C (1,472 °F) may be necessary for PFAS levels to be sufficiently lowered.
A significant aspect of this method is its carbon balance. While the high temperatures needed for pyrolysis and transporting biochar to other locations generate greenhouse gas emissions, the enhanced weathering process helps to offset these emissions. Following the researchers’ guidelines precisely, they estimate that implementing this process on a national scale in the US could remove 10.5 million metric tons of carbon dioxide from the atmosphere annually.
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“This is a real pathway to actually give farmers in this situation some agency over their land and leave the soil in better condition than it was before,” Thompson says.
The research was published in PNAS.
This article was fact-checked by Clare Watson and edited by Peter Dockrill. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.
